GROWTH AND YIELD OF A SUGARCANE PLANT CROP UNDER WATER STRESS IMPOSED THROUGH DEFICIT DRIP IRRIGATION

Transcription

1 REFEREED PAPER GROWTH AND YIELD OF A SUGARCANE PLANT CROP UNDER WATER STRESS IMPOSED THROUGH DEFICIT DRIP IRRIGATION ROSSLER RL 1,2, SINGELS A 1,2, OLIVIER FC 1 AND STEYN JM 2 1 South African Sugarcane Research Institute, P/Bag X02, Mount Edgecombe, 4300, South Africa 2 Department of Plant Production and Soil Science, University of Pretoria, Private Bag X20, Hatfield, 0028, South Africa ryan.rossler@sugar.org.za Abstract Little is known about the way sugarcane yield is affected by water stress during different phases of crop development. This information is necessary to optimise the allocation of limited water for irrigation. A drip irrigated field experiment of cultivar N49 (a plant crop) was conducted in Komatipoort. Treatments included maintaining available soil water (ASW) between 30 and 60% of capacity during the tillering phase (T), stalk elongation phase (SE) and through both (T+SE), while the ASW was maintained above 60% of capacity in the wellwatered control (WW). The WW and T treatments received 1142 and 985 mm of irrigation, respectively, and experienced few days with stress (ASW <50%), while the soil water potential (SWP) fluctuated between -5 and -40 kpa. The SE (809 mm) and T+SE (633 mm) treatments received much less irrigation and went through 62 and 86 days of stress, respectively, while SWP fluctuated between -10 and -90 kpa. Average cane yield at the final harvest (11 months) of the unstressed treatments was 124 t/ha. Water stress during the stalk elongation phase reduced cane yield by 6 t ha -1 and 11 t ha -1 in the SE and T+SE treatments, respectively. Results showed that the small reduction was due to resurgence in stalk elongation rates after a wetting event. The compensatory stalk growth allowed plants in the stressed treatments to maintain an average stalk growth rate similar to the WW treatment. The findings of this study indicate that reasonable economic yields (>90% of potential) are achievable provided the stress periods are short (<5 days) and mild (SWP >-80 kpa and ASW >30% of capacity). Further research is required to test the applicability of the results on a ratoon crop, on different cultivars and soils, and in areas with different climates. Keywords: cane yield, stalk growth, deficit irrigation, stalk elongation rate, water stress, soil water potential Introduction Irrigation water supply is often less than crops require in fully irrigated areas of South Africa. For example, for the past six and three out of six seasons, irrigation water supply from the Komati and Lomati Rivers (Mpumalanga Province, South Africa) ( 1 personal communication) was below sugarcane irrigation water demand as calculated by the MyCanesim sugarcane model (Singels, 2007). In this situation, sugarcane growers have to make tactical decisions regarding the allocation of limited water to the different fields on their farms. These fields often have a sugarcane crop at different development phases, and possibly with different soils 1 A van der Merwe, TSB Sugar, Komatipoort. South Africa 170

2 and water status. The impacts of applying less water than the crop requires will therefore differ. These decisions are extremely complex and require knowledge of how water stress at different phases of crop development affects yields. Sugarcane develops through the four phases of germination, tillering, stalk elongation and maturation. During the germination phase, primary shoots emerge from vegetative buds found at the nodes of planted setts, or from nodes of the remaining stalk after harvest. Vegetative buds at the nodes of the primary shoot give rise to 6-8 secondary shoots (i.e. tillers), which may in turn produce tertiary shoots (van Dillewijn, 1952). This process continues until peak tiller population is reached. The tillering phase therefore can be defined as the period between the emergence of primary shoots to the occurrence of peak tiller population. From this point, some tillers senesce due to competition for solar radiation (Inman-Bamber, 1994; Bakker, 1999), while the remaining tillers elongate into harvestable stalks. Visible elongation of stalks above the ground surface commences after about eight leaves have appeared. For primary tillers this occurs before the time of peak tiller population, but for many of the lower order tillers this will occur at, or after, peak population occurs. Stalk elongation continues until the crop is harvested. In this study, the time of peak population was taken as the end of the tillering phase and start of the stalk elongation phase. Towards the end the stalk elongation phase the elongation rate of stalks slows and sucrose accumulates in stalks (i.e. maturation phase). According to Doorenbos and Kassam (1979) yield is most sensitive to the occurrence of a water stress during the germination and tillering phases, followed by the stalk elongation phase, and least sensitive to water stress is the maturation phase. Pene and Edi (1999) reported results that showed that yield is most sensitive to a water stress during the stalk elongation phase, as crops can recover from a water stress during the tillering phase. Robertson et al. (1999) also found that crops were able to recover from a water stress during the tillering phase through increased tiller and leaf emergence rates (i.e. re-establishing the canopy), provided the stress was not too severe and did not continue for too long. In a study by Wiedenfeld (2000), yield was not affected by a water stress which was imposed by withholding irrigation for six weeks during the stalk elongation phase. Robertson et al. (1999) also withheld irrigation during the stalk elongation phase but for a longer duration (two to three months) and reported significant yield reductions. Reports of widely different crop responses are likely due to the wide range of water stress severity and durations imposed in each study. Many studies (Robertson and Donaldson, 1998; Inman-Bamber, 2004) have shown that water stress during the maturation phase (i.e. the practice of drying-off) increases sucrose yields provided the water stress is not too severe. It is clear that considerable uncertainty exists regarding crop response to water stress in different developmental phases and that more research is required, especially on how yield is affected by mild water stress under deficit irrigation during different phases. The aim of this study was to investigate the response of sugarcane cultivar N49 to a mild water stress during the tillering and stalk elongation phases. Although data from both phases will be reported, it was not possible to effect a stress in the tillering phase, and the paper will therefore focus on the stalk elongation phase. Crop development, growth and yield were related to crop and soil water status. This information is needed to develop tools for optimising limited irrigation water. 171

3 Methods Site and soils A drip irrigated field trial was conducted on the South African Sugarcane Research Institute (SASRI) Mpumalanga Research Station near Komatipoort (25 37 S, E, 187 masl). Cultivar N49 was planted in dual rows (centres spaced at 2 m, individual rows at 0.6 and 1.4 m) on 8 November 2011 and harvested on 10 October 2012 (11 month growing cycle). Cultivar N49 was chosen because it is not prone to flowering or lodging and there is growing interest from growers to plant this variety. Each dual row had a surface dripper line with emitters spaced at 0.6 m. Standard cultivation, fertiliser application and weed control practices were followed. The sandy clay loam (37% clay, 16% silt, 47% sand) had a field capacity (FC) and permanent wilting point (PWP) of 165 and 94 mm, respectively, in the assumed root zone of m, giving this soil the capacity to hold 71 mm of plant available water (ASWC). These values are similar to those found for the same field by Olivier et al. (2006). FC was determined by measuring the volumetric soil water content in the root zone (SWC, in mm) with a neutron water meter (NWM; Model 503DR CPN Hydroprobe, Campbell Pacific Nuclear, CA, USA) two days after an infield calibration plot was saturated and covered with plastic. PWP was determined by measuring SWC with a NWM after all plant available water was extracted by a fully canopied crop (negligible change in SWC over time). Treatments Four irrigation treatments were applied with the aim of maintaining plant available soil water (ASW) between 30 and 60% of ASWC through (1) the tillering phase (T), (2) the stalk elongation phase (SE) and (3) through both tillering and stalk elongation phases (T+SE), while the ASW was maintained above 60% of ASWC in the well-watered control (WW) and during development phases where water stress was not imposed. Treatments were replicated five times in a completely randomised block design. Plots were 12x20 m in size and had six dual rows each. Three weeks prior to harvest all treatments were irrigated to fill the soil profile, and thereafter irrigation was withheld (i.e. drying-off period) for three weeks, following Donaldson and Bezuidenhout (2000). Measurements SWC was measured with a NWM three times a week underneath the dripper line halfway between emitters, at 0.15 m depth intervals, commencing at a depth of 0.25 m to a maximum depth of 0.55 m. ASW was calculated as the difference between measured SWC and PWP. Soil water potential (SWP) was measured between emitters in close proximity to NWM access tubes at depths of 0.25 and 0.44 m using tensiometers (CFM Industries). It should be noted that the soil water regime underneath drip emitters is likely to be wetter, and in the interrow drier, than that monitored. Daily crop water use (CWU) and number of stress days (defined as a day when simulated ASW was below 50% of capacity) was estimated using the MyCanesim sugarcane model (Singels, 2007). Actual irrigation and local weather data were used as inputs, and simulated ASW was corrected with measured values of ASW. This method of determining CWU was preferred over a water balance approach using measured ASW values, because frequent 172

4 drainage events due to rainfall made it impossible to calculate reliable values of CWU through the water balance approach. Stalk height (distance from the ground to the top visible dewlap) was measured twice a week on eight tagged stalks in three plots per treatment. Average stalk elongation rate (SER in cm/d) was calculated as the change in average stalk height between two consecutive measurements divided by the number of days between these measurements. Relative SER (RSER) was calculated as the average SER of stalks in the stress treatments relative to the average SER of stalks in the control treatment. The number of dead leaves (more than 90% of leaf area necrotic) and the number of fully expanded green leaves were counted fortnightly on each of these tagged stalks. Stalk population was determined in a demarcated 5 m section in three plots per treatment. Fractional interception of photosynthetic active radiation (PAR) was measured fortnightly in three plots per treatment, using a ceptometer (AccuPAR LP-80, Decagon Devices, Pullman, WA, USA). At the end of the tillering phase and at final harvest, all stalks within 1.5 m of a dual row were harvested and partitioned into millable stalks, leaves (defined as tops, green laminae and sheaths) and trash (defined as dead leaves and stalks) and the mass of each component determined. The dry mass of each component was determined after subsamples were dried in an oven until a constant mass was reached. Stalk subsamples (16 stalks per plot) were analysed for sucrose, fibre and dry matter content using the method described by Singels et al. (2005). The green leaf area of a fresh leaf subsample of about 1 kg was measured using an area meter (Li-Cor 3100, LI-COR, Nebraska, USA) to determine specific leaf density. This was used to estimate green leaf area index (GLAI in m 2 /m 2 ) from fresh leaf mass data. At the final harvest, millable stalk fresh biomass (cane yield) was determined by weighing the cane harvested from the total net area (186 m 2 ) of the four inner dual rows of each plot. Water relations Results Available soil water and soil water potential trends During the tiller phase the T and T+SE treatments received little irrigation (Table 1), but several large rainfall events prevented ASW from declining into the targeted range (40-60% of ASWC) and the SWP at 400 mm from declining below -30 kpa (Figure 1a,b and Figure 2a,b). Therefore treatments used similar amounts of water and experienced no water stress during this phase (Table 1). During the stalk elongation phase the stressed treatments received about 320 mm (40%) less irrigation than the WW treatment, resulting in the desired ASW regime for these treatments (Figures 1b,c,d). SWP of the stressed treatments fluctuated between -10 and -90 kpa compared to a range of -5 to -40 kpa for the WW treatments (Figures 2b,c,d). As a result, the SE and T+SE treatments endured more stress than the WW treatment, as reflected by the number of stress days in Table 1. CWU was less affected by water stress, with a reduction of 40 to 58 mm (5 to 8%) in the stressed treatments compared to the unstressed treatments (Table 1). 173

5 Table 1. Phase duration, rainfall, irrigation, number of stress days and estimated crop water use for each treatment during the tillering phase, the stalk elongation phase, the three week drying-off period and the total for the growing season. Development phases Drying- Treatment Stalk Total Tillering off elongation Duration of each phase (days) Rainfall (mm) Irrigation (mm) T SE T+SE WW Stress days T SE T+SE WW Crop water use (mm) T SE T+SE WW It is clear from Figure 1b,c that ASW at times was above 50% of ASWC during stress periods because of irrigations applied to maintain ASW above the lower threshold of 30% of ASWC. Therefore, the duration of individual stress periods (consecutive days of water stress) varied from one to 25 days. The frequency distribution of stress period is summarised in Figure 3. The T+SE treatment endured the longest individual stress period (25 consecutive days), followed by the SE treatment (24 consecutive days) while the longest period of stress endured by the T and WW treatments was only 6 and 7 consecutive days respectively. From 301 to 311 days after planting (DAP), 169 mm of rainfall resulted in the sugarcane lodging in all plots. Stress treatments were terminated at this point to minimise the confounding effect of lodging. 174

6 a b c d Figure 1. Available soil water (ASW) in the T (a), T+SE (b), SE (c) and WW (d) treatments. The blue dotted horizontal line represents field capacity (70 mm). The black horizontal lines represent 30 and 60% of the available soil water capacity. Where the line is solid no water stress was imposed, while the dotted line represents imposed water stress periods. The green and blue bars represent rainfall and irrigation, respectively. 175

7 a b c d Figure 2. Soil water potential measured at a soil depth of 250 mm (blue line) and 400 mm (red line) in the T (a), T+SE (b), SE (c) and WW (d) treatments. The green and blue bars represent rainfall and irrigation, respectively. 176

8 Figure 3. The number of stress events of a given duration during the stalk elongation phase (excluding the drying off period for each treatment. Growth and development The imposed water stress had no significant effect on stalk population, total leaf number or on the LAI (Table 2.). Green leaf number The number of green leaves per stalk at the end of the tillering phase did not differ between treatments because there was no water stress during this phase (Table 2). Water stress during the stalk elongation phase slowed the rate of leaf emergence slightly and raised the rate of leaf senescence (data not shown). This resulted in a slightly lower number of green leaves in the SE and T+SE treatments (6.4 and 6.8 leaves, respectively) compared to the WW treatment (8.5 leaves) towards the end of this phase (Figure 5). However, at the end of the stalk elongation phase (before commencing the drying-off) differences between treatments were not significant. Inman-Bamber (1991) reported that leaves tend to accumulate within the leaf whorl during stress periods, and then rapidly emerge once the stress is relieved. This could have been the case here, because stress periods were regularly interspersed with short periods of no stress during which plants could resume leaf development processes at accelerated rates. 177

9 Table 2. Crop growth parameters (mean±sd) for the different treatments at the end of the tillering and stalk elongation phases. Growth indicators Stalk population (m 2 ) Treatments T SE T+SE WW Significance Tillering phase 22.8 ± ± ± ± 1.8 NS Stalk elongation phase 13.4 ± ± ± ± 0.8 NS Total number of leaves emerged Tillering phase 16.8 ± ± ± ± 1.5 NS Stalk elongation phase 31.6 ± ± ± ± 1.8 NS Total number of dead leaves Tillering phase 6.8 ± ± ± ± 1.8 NS Stalk elongation phase 23.8 ± ± ± ± 1.7 NS Green leaves per stalk Tillering phase 10.0 ± 3.0 b 11.5 ± 1.7 a 10.0 ± 1.4 b 11.5 ± 1.1 a * Stalk elongation phase 8.4 ± ± ± ± 1.4 NS Radiation interception (%) Tillering phase 85.9 ± ± ± ± 3.5 NS Stalk elongation phase 98.7 ± 0.5 a 97.9 ± 0.9 a 96.4 ± 1.2 b 98.2 ± 0.4 a * Green leaf area index (m 2 /m 2 ) Tillering phase 3.96 ± ± ± ± 0.28 NS Stalk elongation phase 3.38 ± ± ± ± 0.46 NS SER per development phase (cm/day) Tillering phase 1.41 ± ± ± ± 0.13 NS Stalk elongation phase 0.70±0.07 a 0.52±0.08 c 0.58±0.06 c 0.63±0.07 b * Stalk height (cm) End of tillering phase 103 ± ± ± ± 8.30 NS At harvest 258 ± ± ± ± 15.2 NS *indicate significance at P 0.05 and NS indicated non-significance between treatments. As expected, the insignificant impact of stress on stalk population, leaf numbers and LAI also resulted in little impact on canopy cover and radiation interception. The only significant difference found was a slightly lower radiation capture for the T+SE treatment compared to the other treatments (Table 2). Stalk elongation The RSER of all treatments declined with declining ASW, while increases in ASW due to rainfall and/or irrigation resulted in a resurgence in RSER (Figure 6). Such fluctuations occurred irrespective of the ASW level, presumably because roots at the top part of the root zone were able to extract enough water for the plant to support high rates of plant processes. 178

10 Figure 5. Number of green leaves per stalk for the different treatments (T, SE, T+SE and WW) during the tillering and stalk elongation phases. Error bars represent the standard deviation of the WW treatment. During the tillering phase the SER of the different treatments was similar, except towards the end of the phase when ASW declined to a value of 43 mm and the RSER of the stressed treatments declined rapidly to 0.5 in response (Figure 6). Although the average SER of the stressed treatments during the tillering phase was lower than that of the unstressed treatments, this difference was not significant, nor were differences in stalk height (Table 2). During the stalk elongation phase RSER declined rapidly to about 0.5 whenever ASW declined below the 60% capacity. When ASW increased due to rainfall and/or irrigation, RSER quickly recovered to and sometimes exceeded a value of 1, suggesting that stalks were able to compensate for the slow growth during the stress period (Figure 6). Inman-Bamber and de Jager (1986) reported similar results in a potted sugarcane trial, where a stressed crop s SER exceeded that of an unstressed crop soon after stress was relieved. The result of this compensatory growth was ascribed to cells regaining turgor pressure quickly (Inman- Bamber, 1995). RSER did not recover after wetting events after about 250 DAP for the SE treatment, and after about 200 DAP for the T+SE treatment. This suggests that a prolonged period of intermittent stress may eventually damage the plant s ability to compensate. Average RSER for the stalk elongation phase of the stressed treatments was about 0.1 cm/day (9-17%) lower than that of the unstressed treatments (Table 2). This lower growth rate resulted in stalks being 7% shorter than those in the unstressed treatments at final harvest, although this difference was not significant. 179

11 a b c Figure 6. Relative stalk elongation rate and available soil water for T (a), T+SE (b) and SE (c) treatments during the tillering and stalks elongation phases. The black horizontal dotted lines indicate RSER = 1. Biomass partitioning 180

12 At the end of the tillering phase the T+SE treatment had significantly lower leaf dry biomass and higher trash biomass than the other treatments (Table 3). It is unclear why this treatment was different from the others, as no stress occurred during the tillering phase. Water stress during the stalk elongation phase significantly reduced the dry mass of leaves in the SE and T+SE treatments by 1.4 to 1.5 t/ha, but had no significant effect on millable stalk dry mass (4.4 to 6.1 t/ha reduction) or trash dry mass (0.6 to 1.7 t/ha reduction) (Table 3). Partitioning of biomass was not affected by water stress. The imposed water stress during the stalk elongation phase reduced cane yield by a statistically significant 6 t/ha and 11 t/ha in the SE and T+SE treatments, respectively. Sucrose yields dropped by 0.4 to 1 t ha -1 but these reductions were not significant (Table 3). Crop stage Table 3 Dry mass (in t/ha and as percentage of the total) of biomass components for each treatment at the end of the tillering phase and at final harvest, and fresh cane and sucrose yields at harvest. Component Treatments T SE T+SE WW Significance End of tillering phase (t/ha) Stalk 3.84 ± ± ± ± 0.64 NS Leaves 6.07 ± 0.86 b 5.77±0.84 ab 4.73 ± 0.76 a 6.64 ± 0.88 b * Trash 0.19±0.06 ab 0.12 ± 0.03 a 0.25 ± 0.09 b 0.14 ± 0.03 a * Total 10.09± ± ± ±1.40 NS End of tillering phase (%) Stalk 37.8 ± ± ± ± 3.25 NS Leaves 60.2 ± ± ± ± 3.25 NS Trash 1.92 ± 0.59 a 1.31 ± 0.28 a 3.1 ± 0.86 b 1.28 ± 0.15 a * Final harvest (t/ha) Stalk 34.5 ± ± ± ± 3.32 NS Leaves 6.77 ± 1.24 b 5.08 ± 1.36 a 5.13 ± 0.75 a 6.57 ± 0.60 b * Trash 10.54± ± ± ±2.14 NS Total 51.8 ± ± ± ± 5.54 NS Final harvest (%) Stalk 66.7 ± ± ± ± 2.33 NS Leaves 13.0 ± ± ± ± 0.48 NS Trash 20.3 ± ± ± ± 2.45 NS Cane yield (t/ha) 124 ± 5.3 b 117 ± 7.2 a 112 ± 6.1 a 123 ± 1.7 b * Sucrose yield (t/ha) 18.4 ± ± ± ± 1.7 NS *indicates significance at P 0.05, and NS indicated non-significance between treatments 181

13 Concluding discussion In this study water stress during the stalk elongation phase imposed through deficit irrigation (60% of requirement) had no significant long lasting effects on sugarcane growth and development processes. The stress had no effect on leaf emergence and senescence rates or on the number of green leaves per stalk. Stalk population, GLAI and radiation interception were also not affected. Stalk elongation rate was found to be highly sensitive to changes in ASW, declining rapidly with declining ASW. However, plants appear to have the ability to compensate to some extent for growth lost during short periods of stress, through accelerated growth when stress is relieved. On average, however, stalk elongation rate was reduced by about 0.1 cm/day, which resulted in water stressed stalks being 7% shorter than unstressed treatments. Crop water use was not affected much by the deficit irrigation, with a reduction of about 4%. Water stress during the stalk elongation phase reduced cane yield by 6 to 11 t ha -1 (5 to 9 %) and sucrose yield by 0.4 to 1 t ha -1 (3 to 5 %). The small size of the yield losses are partially attributed to the compensatory growth of stalks during frequent periods when stress was relieved. In other studies (Robertson et al., 1999; Pene and Edi, 1999) much larger reductions in yield were observed when drought stress was more severe. The reductions in cane and sucrose yields found in the current study are similar to the yield reduction of 4 t/ha found by Pene and Edi (1999) when irrigation was scheduled according to 75% of Class A-pan evaporation. Results suggest that sugarcane can achieve reasonable economic yields (>90% of potential) with deficit drip irrigation, provided the stress periods are short (<5 days) and mild (SWP >- 80 kpa). It is necessary to schedule irrigation accurately to successfully maintain the soil water status in the desired regime and ensure continued water use and stalk growth. However, further research is required to test the applicability of the results on a ratoon crop, with different cultivars and soils, and in areas with different climates. Placement of monitoring instruments relative to drip emitters and cane rows also requires further investigation. Acknowledgements The authors would like to thank the SASRI technical team for their support and SASRI for funding the project. REFERENCES Bakker H (1999). Sugarcane Cultivation and Management. Kluwer Academic/Plenum Publishers, New York, USA. Donaldson RA and Bezuidenhout CN (2000). Determining the maximum drying off periods for sugarcane grown in different regions of the South African industry. Proc S Afr Sug Technol Ass 74: Doorenbos J and Kassam AH (1979). Yield response to water. FAO Irrigation and Drainage paper No. 33. Food and Agricultural Organisation of the United Nations, Rome. 182

14 Inman-Bamber NG (1991). A growth model for sugar-cane based on a simple carbon balance and the CERES-Maize water balance. S Afr J Plant Soil 8: Inman-Bamber NG (1994). Temperature and seasonal effects on canopy development and light interception of sugarcane. Field Crops Res 36: Inman-Bamber NG (1995). Automatic plant extension measurement in sugarcane in relation to temperature and soil moisture. Field Crops Res 42: Inman-Bamber NG (2004). Sugarcane water stress criteria for irrigation and drying off. Field Crops Res 89: Inman-Bamber NG and de Jager JM (1986). The reaction of two varieties of sugarcane to water stress. Field Crops Res 14: Olivier FC, Donaldson RA and Singels A (2006). Drying of sugarcane on soils with low water holding capacity. Proc S Afr Sug Technol Ass 80: Pene CBG and Edi GK (1999). Sugarcane yield response to deficit irrigation at two growth stages. pp In: Kirda C, Moutonette P, Hera C and Nielsen DR (Eds), Crop Yield Response to Deficit Irrigation. Kluwer Academic Publishers, Doordrecht, Boston, London. Robertson MJ and Donaldson RA (1998). Changes in the components and sucrose yield in response to drying-off of sugarcane before harvesting. Field Crops Res 55: Robertson MJ, Inman-Bamber NG, Muchow RC and Wood AW (1999). Physiology and productivity of sugarcane with early and mid-season water deficit. Field Crops Res 64: Singels A, Donaldson RA and Smit MA (2005). Improving biomass production and partitioning in sugarcane: Theory and practice. Field Crops Res 92: Singels A (2007). A new approach to implementing computer-based decision support for sugarcane farmers and extension staff. The case of My Canesim. Proc Int Soc Sug Cane Technol 26: van Dillewijn C (1952). Botany of Sugarcane. H Veenaman and Zonen, Wageningen, Netherlands. Wiedenfeld RP (2000). Water stress during different sugarcane growth periods on yield and response to N fertilization. Agric Water Manage 43: